– Marketing: According to this article in The Independant RBR is a cash cow, because it's revenue (for the brand Red Bull) is bigger than their sponsoring investment (that means paying for the same amount of TV appearance that they get from F1 feeds over the world would be more expensive).
Red Bull's Advertising Value Equivalency – the price it would have to pay to buy a similar amount of on-screen exposure – came to an estimated £144m in 2009 and £232m in 2010, when it received almost a quarter of the total received by all the teams. It has already got exposure worth an estimated £115m in 2011, so over the past three years alone this has more than offset the cost of Red Bull's investment in F1
The Advertising Value Equivalency of Toro Rosso surely is at a much lower level. Maybe the gains would not be big enough to compensate the loss on RBR's side (on the marketing aspect).
– Marketing for STR: You could choose a time for a rising to be attractive for sponsoring. (Mateschitz is searching for partners, but admits the brand Red Bull is too strong.)
– Driver academy: STR is RBR's new drivers test team. If the car/team struggles it could be a better training for the drivers to learn how to develop a car.
– Concealment: Is RBR secretly testing technology on STR cars or gaining tech data? Then it cold be wise to cut off some downforce so that the contenders don't have a look at a slow car.
At last I'm not convinced of these thoughts. Now that STR is able and willing to fight for 6th in the WDC, why shouldn't they the whole year? The difference 6th to 8th is $9m.